There are many applications wherein liquid crystal displays are utilized to advantage. For example, liquid crystal displays find use in digital watches, digital clocks, calculators, pocket-sized television receivers, and various forms of portable games, to name just a few.
Liquid crystal displays generally include a plurality of pixels (picture elements) arranged in rows and columns. Each pixel includes a pair of electrodes. As is well known, when a voltage is applied across the electrodes, the optical properties of the liquid crystal material can be changed to provide a light or dark display depending upon the type of liquid crystal material used and the desired mode of operation of the display.
To obtain a usable display, the voltage potential across the electrodes of each pixel must be selectively applied. In the prior art, the selective application of these voltage potentials has been accomplished through the use of thin film transistors. While thin film transistors are generally successful in these applications, they can only be used for small area liquid crystal displays. Conventional thin film transistors also exhibit relatively high power dissipation and low frequency operation. This limits the number of pixels which can be driven in a liquid crystal display which in turn limits the liquid crystal display areas.
Another problem in using thin film transistors in liquid crystal displays is yield. Virtually one-hundred percent of all of the thin film transistors must be operational to obtain a usable display. Such yields are difficult to achieve over small display areas and virtually impossible to achieve for large area displays because the making of thin film transistors requires numerous process steps, many of which require extremely accurate photolithography. Accurate or precise photolithography is required to control important physical device dimensions, principal among them being the source to drain spacing which determines, in conventional planar thin film transistors, the length of the current conduction channel between the source and drain. The channel length dramatically effects both the device current and operating frequency limit. As a result, the high yields required cannot be readily achieved for large display areas with conventional photolithography techniques.
The present invention overcomes such deficiencies exhibited by thin film transistors by using diodes to drive the display pixels. Furthermore, the diodes can be formed without the need of precise photolithography and in fewer process steps than that required to form thin film transistors.
The use of diodes to drive displays has been previously proposed. However, previously proposed diode driven displays were limited to discrete diode devices which of course are not appropriate for large area displays employing potentially thousands of pixels. Such displays would be necessarily and unduly complicated both physically to implement and electrically to address the individual pixels.
The present invention overcomes these problems by forming the diodes integrally with the display structure. The diodes can be formed from deposited semiconductor materials by processes not incompatible with the other display processes.